Huntington disease is a fatal nervous system disorder causing degeneration of th

Question

Huntington disease is a fatal nervous system disorder causing degeneration of the brain.  It is caused by a defective gene on chromosome #4.  This is an autosomal dominant trait affecting 1 in 20,000 people. However, it is a delayed action gene that is not expressed until the affected individualis in his or her late 30s or40s.  The homosygous condition is lethal to the fetus.  Using the Hardy-Weinberg equation, determine (1) the frequency of carriers in the population, (2) the frequency of fetuses affected by the lethal homozygosity, and (3) the frequency of unaffected individuals.

Explanation / Answer

From the question let us assume   that     p=dominant allel         &    q = recessive. According to Hardy Weinberg law     Allel   frequency at a loci given by       p+q = 1 Genotype frquency at a locus   is   given by p 2 +  2pq + q 2 = 1     Allel   frequency at a loci given by       p+q = 1 Genotype frquency at a locus   is   given by p 2 +  2pq + q 2 = 1 Using the   Hardy - Weinberg equations Given the frequency   of Huntington disease or p2   =1/20000 = 0.00005    (2)        thus p= p2 =0.00005 = 0.0071approx Therefore frequency of  fetuses affected   by  homozygosity              (3)    From  the   Hardy -Weinberg equation    frequency of   recessive allele = 1-p   = 1- 0.0071=0.9929 Therefore    frequency of unaffected   individuals (1) Frequency   of  the    heterozygotes = 2pq =   2*0.000050* 0.9929 = 0.000099 As   this is a dominant trait   it does not have   carriers , but individuals   which will   be affected   at   the later stage are   heterozygotes . As   this is a dominant trait   it does not have   carriers , but individuals   which will   be affected   at   the later stage are   heterozygotes .                                                           

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